1. Field of the Invention
The present invention relates to a duplex controller having a write through update bus connecting an active processor circuit and a redundant standby processor circuit for updating the memory of the standby processing circuit when the memory of the active processing circuit is updated.
2. Description of the Related Art
The fault tolerance of a data processing system relates to the ability of the system to continue performing a task after the occurrence of a fault. One of the ways in which systems achieve increased fault tolerance is by incorporating hardware redundancy. Fault tolerance is extremely important in systems which require high reliability such as aircraft control systems, life-support systems, and other industrial control systems. Redundant or duplicative data processors are used in controllers to increase the availability and reliability of the systems in which they are used.
Controllers which have standby processors may comprise many different configurations. For example, some systems include a quasi-duplicative arrangement in that only some of the parts of the circuit are duplicated. In other configurations in which high reliability is extremely critical, the entire circuits are duplicated. However, even in those systems in which an entire hardware circuit is duplicated, some amount of time is required to switch from using the active controller to the standby controller. For example, if the standby controller lacks the state information of the failed active side, the system must re-initialize to allow the standby side, which is now the active side, to recreate or reproduce a useful state, which is time consuming. This associated delay deteriorates the availability of the overall device which may include many of these redundant circuits. One way to avoid this delay is to have the standby controller perform ongoing tasks simultaneously with the active controller. In this way, the standby processor is always in the same state as the active processor and can assume control immediately. However, this arrangement consumes approximately double the power of a single circuit and may therefore be prohibitive in many systems in which numerous components have a redundant standby device. Also, these types of circuits normally require additional complex circuitry for ensuring that the active and standby circuits are performing the same operations simultaneously and producing the same results.